The invention relates to a thermal insulating material, which is particularly suited for high temperature applications far above 1000xc2x0 C. and can be employed in gas turbines, aeroplane engines, power station turbines and other highly, thermally loaded parts, for example in vehicle construction and energy technology.
The invention further relates to a method for producing and processing such a thermal insulating material.
The known thermal insulating materials specifically employed for high temperature applications in heat-power machines and in industrial plants are oxide cover layers applied to a metal substrate, for example on a highly alloyed nickel base material in a turbine blade. The classical thermal insulating layer consists of tetragonal or stabilised ZrO2 as a cover layer, which is usually applied to an additional intermediate layer in the form of a low melting point or soft coupling layer (HVS). The coupling layer is composed substantially of aluminum and yttrium, frequently also with amounts of platinum and palladium (up to 10 wt.-%), apart from further components (nickel, chromium, cobalt), to make it more oxidation resistant. The ceramic cover layer is most often applied by atmospheric plasma spraying (APS). Newer developments concern ZrO2 layers vapour-deposited with electron beams (electron beamsxe2x80x94physical vapour deposition, EB-PVD-ZrO2-layers). The requirements on the ceramic ZrO2 cover layer and the coupling layer have increased continuously in recent years. Their stability under alternating temperatures, their protective effect against oxidation as well as their long-term stability and adhesion at higher temperatures of the exhaust gas for increased efficiency have been optimised.
As a disadvantage of the known thermal insulating layers on the basis of ZrO2 it has been found that layers applied by plasma spraying or CDV and EB-PVD layers of stabilised ZrO2 are not sufficiently resistant above 1100xc2x0 C. The ZrO2 layers age rapidly at temperatures above 1100xc2x0 C.
This aging process leads to a partial densification of the layer and parallel to that the elasticity modulus of the layer increases. The density increase diminishes the original uniform fine porosity of the layer and the thermal conductivity increases. The increase in elasticity modulus of the ceramic layer means that the thermal shock resistance decreases and the xe2x80x9ctolerancexe2x80x9d or the capability of compensating for thermal expansion with highly different thermal expansion coefficients between the ceramic layer and the metallic substrate decreases. Both processes, the density increase and the increase in elasticity modulus lead to a peeling of the ZrO2 cover layer during the temperature cycles in a turbine.
In addition to deterioration the pure mechanical properties of the cover layer, the three dimensional sintering of the ZrO2 layer leads to the formation of a dense ceramic with other properties than that of the porous layer. Since ZrO2 is a very good conductor of ions, the ever present oxidative degradation in the entire ceramic-metal composite is not altered by densification of the ceramic. The coupling layer oxidises in this process and a layer of oxidation products with other properties forms between the original coupling layer and the ceramic cover layer. The original ceramic layer thus in the end breaks up due to the altered mechanical properties of the layer system. The corrosion of the coupling layer continues despite the sometimes very dense ceramic surface.
The object of the present invention is therefore to provide an improved thermal insulating material which is better suited for high-temperature applications and is particularly suited for coating turbine blades and similar high temperature components.
Furthermore, a suitable method for producing and processing such thermal insulating materials is to be provided.
According to the invention, this object is solved by a thermal insulating material composed of a first component with at least one first phase containing stoichiometrically 1 to 80 mol-% of M2O3, 0 to 80mol-% MeO and a remainder of Al2O3 with incidental impurities, wherein M is selected from the elements lanthanum and neodymium or mixtures thereof and wherein Me is selected from zinc, alkaline earth metals, transition metals, and the rare earths or mixtures thereof, preferably selected from magnesium, zinc, cobalt, manganese, iron, nickel, chromium, europium, samarium or mixtures thereof.
An effective thermal insulation is made possible with the insulating material of the invention, also at temperatures of 1300xc2x0 C. and up to over 1500xc2x0 C., whereby at the same time sintering processes and the resultant ageing and grain enlargement compared to ZrO2 are greatly slowed down or retarded.
In a preferred embodiment of the invention, the first component contains 1 to 80 mol-% M2O3 and 0.5 to 80 mol-% MeO with a remainder of Al2O3.
It has been shown to be of advantage when the first component comprises 1 to 50 mol-% M2O3 and 1 to 50 mol-% MeO with a remainder of Al2O3.
It is further preferred when the first component comprises 1 to 20 mol-% M2O3 and 2 to 30 mol-% MeO with a remainder of Al2O3.
Furthermore, it has been shown to be of advantage when the first component comprises 2 to 20 mol-% M2O3 and 5 to 25 mol-% MeO with a remainder of Al2O3.
Particularly preferred is the first component comprising 5 to 10 mol-% M2O3, about 10 to 20 mol-% MeO with a remainder of Al2O3.
Particularly advantageous properties result when the first component comprises about 5 to 9 mol-% M2O3, about 12-17 mol-% MeO with a remainder of Al2O3, whereby a composition with about 7.1 mol-% M2O3, about 14.3 mol-% MeO and a remainder of Al2O3 represents an optimal composition.
The first phase preferably forms a hexa-aluminate phase of magnetoplumbite structure of the composition MMeAl11O19, which when using lanthanum as M and magnesium as Me is known as magnesium aluminum lanthanum oxide with the formula MgAl11LaO19.
This material consists mainly of aluminum oxide in which monolayers of lanthanum oxide and aluminum oxide are disposed at regular spacings. This insertion of La2O3 leads to the formation of a layered structure with a characteristic plate-like structure of the crystals. This magnetoplumbite phase only forms in a narrowly restricted composition region. The typical composition LaAl11O19 due to its structure has very many cationic (about 8% Al) and anionic (about 5% O) vacancies in the lattice, which allow the diffusion of atoms through the structure. The homogeneity region of the phase is extended to LaMgAl11O19by doping with bivalent cations having a small ionic radius (typically Mg++, Mn++, Co++, Zn++, etc.). In this ideal composition LaMgAl11O19 the compound has nearly no more possibility of altering its composition.
With a further increase in the doping with MgO and La2O3 (or MeO and M2O3) further defects form in the structure and a multiphase region forms including LaMgAl11O19, MgAl2O4, LaAlO3 and MgO.
In the optimal composition according to the invention, the addition of MeO leads to a decrease in the lattice vacancies. This means that the material with the composition LaMgAl11O19 (MMeAl11O19) has absolutely no more crystal defects in the structure or formulated in another way, all of the vacancies in the structure are occupied by Me (Mg) and an additional O atom. This complete occupancy of all lattice sites in the structure leads to the desired high thermo-chemical stability and phase stability in the temperature region above 1100xc2x0 C.
A further important advantage of the thermal insulating material of the invention is that the material is substantially inert with respect to alkali compounds (Na2O, NaCl, K2O, KCl) of the combustion gas or the surrounding atmosphere.
Previous thermal insulating materials based on ZrO2 form low melting point phases with the hydroxides or carbonates of Na2O and K2O or with the NaCl contained in the atmosphere in winter or near the sea, which lead to a strong densification of the sprayed layer at temperatures of 1000xc2x0 C. In contrast, such attacks on the thermal insulating material of the present invention lead more to an increased plate growth, which subsequently makes densification, i.e. the sintering of the cover layer substantially more difficult.
A further advantage of the thermal insulating material of the present invention is a favourable thermal expansion coefficient, which lies between 9.5 and 10.7xc3x9710xe2x88x926 [Kxe2x88x921] in a temperature range between room temperature and 1200xc2x0 C. and thus in a range favourable for coating highly heat resistant steels, which have an expansion coefficient of about 10 to 12xc3x9710xe2x88x926 [Kxe2x88x921].
With the thermal insulating material of the present invention, the application of a thin, very effective thermal insulating layer is possible on a body, for example made of chromium nickel steels, which have an exceptionally high temperature resistance and long-term stability and by which a peeling of the thermal insulating layer from the base material is effectively avoided even after numerous thermal cycles.
The thermal insulating material of the present invention is preferably applied by thermal spraying, in particular by plasma spraying as a thermal insulating layer on a body to be coated.
To achieve a preferred crystallisation of the aluminate during the plasma spraying and to increase adhesion and thermal shock resistance, the material can additionally comprise a second component which preferably is substantially insoluble in the hexa-aluminate phase and preferably is added to the first component in an amount of about 0.001 to 20 wt.-%, in particular about 0.1 to 10 wt.-%, whereby the range of 0.1 to about 3 wt.-% is particularly preferred.
The second component can comprise at least one of the compounds ZrO2 in monoclinic, tetragonal or cubic form, La2Zr2O7, MgZrO3, Nd2O3, HfO2, Y2O3, Yb2O3, Eu2O3, La2Hf2O7, MgHfO3, oxides or salts of the alkali metals sodium, potassium, lithium or mixtures or alloys of these compounds.
If ZrO2 is added in tetragonal or cubic form, then preferably it is doped with MgO, CaO or Y2O3.
Concerning the salts of the alkali metals sodium, potassium and lithium, which can also be added as a doping of the first component, these can be carbonates, chlorides, nitrates, acetates, formates, citrates, sulphates, hydrogen carbonates or mixed salts of these metals.
The thermal insulating material of the present invention is preferably first produced in powder form and then subsequently applied as a thermal insulating material to a component, for example by plasma spraying, or is processed to produced massive components using powder technology methods or is further processed to a ceramic foam.
According to a first alternative, the powder-like thermal insulating material is produced by adding an insoluble oxide, a hydroxide or an oxygen hydrate of Al2O3 as the starting material to an aqueous or alcoholic medium, in particular methanol, ethanol or isopropanol, the remaining portions of the first component being soluble salts, preferably carbonates, hydrogen carbonates or acetates. The starting material is dissolved in the medium, the formed suspension is dried, preferably after a grinding and dispersion step, preferably spray dried, and the resulting powder is subsequently subjected to an annealing treatment.
A relatively uniform distribution and a good mixing of the various additives is achieved in this wet chemistry process, by which an insoluble carrier powder is coated. The subsequent annealing treatment is preferably carried out at temperatures of 500 to 1800xc2x0 C. in the presence of air for a duration between about 0.5 and 20 hours. The annealing process, for example in a rotary oven, produces a single-phase, oxidic agglomerate with an average diameter of between about 1 and 200 xcexcm and with a specific surface area between 0.1 and 40 m2/g.
According to a second alternative for the powder production, the compounds of the first component are mixed in powder form as oxides or salts in a mixer, preferably a drum or tumbling grinder, where preferably grinding bodies of Al2O3 or stabilised ZrO2 are employed. The powder is subsequently granulated and subjected to an annealing treatment.
This so-called xe2x80x9cmixed oxide methodxe2x80x9d is the simplest variation for production, however, it is somewhat more difficult to obtain a homogeneous mixture. At first the produced powder still has several phases even after the mixing process.
The multiphase oxide mixture is preferably treated with binders and granulated before the annealing treatment is carried out, which is also preferably performed in the presence of air, preferably for a duration between 0.5 and 20 hours in a temperature range of between about 300xc2x0 C. and 1800xc2x0 C.
A homogeneous oxidised powder is formed by the annealing process, where the granulates have an average diameter of between about 1 and 200 xcexcm and a specific surface area between 0.1 and 40 m2/g.
Conversely, if the mixing is performed as mentioned above in a liquid medium or one works with a suspension having a high content on solids, then a drying is initially carried out, preferably by spray drying, before the subsequent annealing treatment.
A third alternative for producing the powder-like thermal insulating material is producing the powder by a sol-gel process with subsequent drying and annealing.
A particularly good chemical homogeneity and a complete phase transition during the annealing is achieved when using a sol-gel process. The powder produced by the sol-gel process is particularly fine grained and is well suited for subsequent processing by powder technology methods or by plasma spraying.
In the sol-gel process, alcoholates are preferably produced from the starting materials in the desired mass ratios, subsequently solid components are precipitated out of the solution, preferably by the addition of water or by pH adjustment. The solid components are subsequently separated from the excess solution and dried and then annealed at temperatures between about 500xc2x0 C. and 1200xc2x0 C.
In a variation of this process, organic binding agents are additionally added after the precipitation of the solid components and then the subsequent separation of excess solution takes place, before the drying, preferably spray drying, and finally the annealing treatment follows. The annealing is preferably performed at temperatures between 500xc2x0 C. and 1200xc2x0 C.
In both variations, alcoholate compounds of the form (xe2x80x94OCnH2n+1) are used, whereby xe2x80x94OCnH2n+1 means methoxy, ethoxy, isopropoxy, propoxy, butoxy or isobutoxy alcoholates with 1xe2x89xa6nxe2x89xa65.
Alternatively, water soluble salts of M (lanthanum or neodymium) or Me (in particular magnesium), preferably as an acetate, citrate, carbonate, hydrogen carbonate, formate, hydroxide or nitrate, can be added to a solution of aluminum alcoholate and subsequently precipitated.
If the material comprises a second component, this takes place according to a further embodiment of the invention in the liquid state, in which the second component is added in soluble form before the drying or precipitation takes place (as long as the sol-gel process is being used).
In contrast, in the dry method (mixed oxide method) the compounds of the second component are added as a powder and annealed together with the other compounds and are brought to chemical reaction in the solid phase.
As mentioned above, the thermal insulating material of the present invention can be applied either in powder form by plasma spraying onto the part to be coated or can be subsequently processed using powder technology methods, for example by axial cold pressing, isostatic cold pressing or slip casting and subsequent sintering, preferably under a slightly reducing atmosphere at temperatures of at least about 1500xc2x0 C. or by extruding or casting foils with the corresponding subsequent heat treatment to produce larger articles.
According to another variation of the invention, the powder can also be produced in a ceramic foam, namely by filling a polymer foam with slip whereafter the solvent is preferably evaporated at temperatures between 200xc2x0 C. and 400xc2x0 C. or by adding a suspension of the powder to a low viscosity polymer, which is then foamed with a foaming agent, and finally in both variations carrying out an annealing treatment, preferably at first in a range between 900xc2x0 C. and 1100xc2x0 C. and finally at about 1400xc2x0 C. to 1700xc2x0 C.
It will be understood that the features of the invention are not only applicable in the given combinations but may also be used in other combinations or taken alone without departing from the scope of the invention.